This gist contains the model, service, handler, and route to filter products

filter products

// model

// FilterProduct defines what information may be provided to make a request to get Products and filter the results.
// All fields are optional so clients can send just the fields they want changed.
// It uses pointer fields so we can differentiate between a field that was not provided and a field that was provided as explicitly blank.
// Normally we do not want to use pointers to basic types but we make exceptions around marshalling/unmarshalling.
type FilterProduct struct {
	Name     *string `json:"name"`
	Cost     *int    `json:"cost" validate:"omitempty,gte=0"`
	Quantity *int    `json:"quantity" validate:"omitempty,gte=1"`
}

// service

// FilterProducts gets all the Products from the database that meet the filter criteria.
// Then encodes them in a response client.
// It returns an empty array if none of the Products fit the criteria.
func FilterProducts(ctx context.Context, db *sqlx.DB, fp FilterProduct) ([]Product, error) {

	list := []Product{}

	var (
		name string
		cost,
		quantity int
	)
	if fp.Name != nil {
		name = *fp.Name
	}

	if fp.Cost != nil {
		cost = *fp.Cost
	}

	if fp.Quantity != nil {
		quantity = *fp.Quantity
	}

	const q = `SELECT
		p.product_id, p.name, p.cost, p.quantity,
		FROM products AS p
		WHERE p.Name = $1 OR
		WHERE p.cost = $2 OR
		WHERE p.quantity = $3
		GROUP BY p.Name`

	if err := db.GetContext(ctx, &list, q, name, cost, quantity); err != nil {

		if err == sql.ErrNoRows {
			return nil, ErrNotFound
		}
		return nil, err
	}

	return list, nil

}
// handler

// FilterProducts gets all filtered products from the service layer.
func (p *Product) FilterProducts(ctx context.Context, w http.ResponseWriter, r *http.Request) error {

	ctx, span := trace.StartSpan(ctx, "handlers.Product.FilterProducts")
	defer span.End()

	filterProduct := product.FilterProduct{}

	if err := web.Decode(r, &filterProduct); err != nil {
		return err
	}

	list, err := product.FilterProducts(ctx, p.DB, filterProduct)
	if err != nil {
		return errors.Wrapf(err, "filtering products %q", filterProduct)
	}

	return web.Respond(ctx, w, list, http.StatusOK)
}
// route

app.Handle(http.MethodPost, "/v1/products/filter", p.FilterProducts)